You won't add this table to your solution, you wouldn't want to customize it at all. When you publish your solution to a production environment the refecnes to this table iwill hook up to the Process Stage table in that environment.
I'm going to go out on a limb and asusme you want to interact with this table to add a lookup to the active stage in a custom table or column of yours, and this is definately possible.
To add a lookup for the Process Stage table, you have to use the classic interface; you can't do it in the modern experience.
These screenshots should help a bit.
If you are not familiar at all with the classic interface, don't let it scare you because it is mostly the same but with the old names. Tables are called entities and columns are called fields.
In my case, I want to add a custom table to track the history of every BPF stage, so I created a table called Stage History and in order to add a lookup for the active stage of my BPF process stage, I had to go into the classic experience as below.

